Data-driven enhancement of coherent structure-based models for predicting instantaneous wall turbulence

TL;DR

This paper introduces a methodology to extract 3-D geometries of significant eddies from wall-turbulence data, improving coherent structure-based models' predictions of wall-bounded flows and proposing flow control strategies.

Contribution

It develops a data-driven approach to incorporate 3-D eddy geometries into models, extending the attached eddy model to large-scale motions and enhancing flow prediction accuracy.

Findings

Identification of geometric self-similarity in large-scale motions

Validation of hairpin packets as representative structures across flows

Enhanced flow predictions using empirically derived scalings

Abstract

Predictions of the spatial representation of instantaneous wall-bounded flows, via coherent structure-based models, are highly sensitive to the geometry of the representative structures employed by them. In this study, we propose a methodology to extract the three-dimensional (3-D) geometry of the statistically significant eddies from multi-point wall-turbulence datasets, for direct implementation into these models to improve their predictions. The methodology is employed here for reconstructing a 3-D statistical picture of the inertial wall coherent turbulence for all canonical wall-bounded flows, across a decade of friction Reynolds number ($Re_{\tau}$). These structures are responsible for the $Re_{\tau}$-dependence of the skin-friction drag and also facilitate the inner-outer interactions, making them key targets of structure-based models. The empirical analysis brings out the geometric self-similarity of the large-scale wall-coherent motions and also suggests the hairpin packet as the representative flow structure for all wall-bounded flows, thereby aligning with the framework on which the attached eddy model (AEM) is based. The same framework is extended here to also model the very-large-scaled motions, with a consideration of their differences in internal versus external flows. Implementation of the empirically-obtained geometric scalings for these large structures into the AEM is shown to enhance the instantaneous flow predictions for all three velocity components. Finally, an active flow control system driven by the same geometric scalings is conceptualized, towards favourably altering the influence of the wall coherent motions on the skin-friction drag.